1. Field of the Invention
The present invention relates generally to systems and methods for performing cryosurgery. More particularly, the present invention relates to use of a closed refrigeration system for cooling a surgical probe to very low temperatures in an efficient and rapid fashion.
Cryosurgery has been used for a number of years for the treatment and ablation of tissue for a variety of therapeutic purposes. For example, cryosurgical probes have been used for ablation of the endometrial lining of the uterus for the treatment of bleeding and other disorders. Most cryosurgical systems rely on either liquid nitrogen or nitrous oxide as a coolant medium. Liquid nitrogen is advantageous since it can achieve very low temperatures, but requires extensive insulation in order to protect the patient and physician from the liquid nitrogen while it is being introduced to the treatment site. Moreover, since the liquid nitrogen is evaporated and released into the atmosphere, continual replenishment of the liquid nitrogen source is necessary. Nitrous oxide is advantageous since it can be delivered at room temperature, thus reducing the requirement for insulation, and be cooled at the treatment site by Joule-Thomson expansion. The temperatures achieved by nitrous oxide expansion, however, are not nearly as low as those achieved with liquid nitrogen, and the use of nitrous oxide also suffers from the need for continual replenishment.
An improvement over the use of liquid nitrogen and nitrous oxide is proposed in U.S. Pat. No. 5,275,595 to Dobak, III. In particular, it is proposed that a closed refrigeration system be employed together with a counter-current heat exchanger in the probe of a cryosurgical device. The patent asserts that use of particular refrigerant gas mixtures can achieve very low temperatures suitable for many forms of cryosurgery.
The system of Dobak, III, however, suffers from a number of disadvantages. In particular, use of the counter-current heat exchanger shown in the patent requires that system equilibrium be established before the lowest system temperatures can be achieved. Thus, the system must be running for some time before useful temperatures can be achieved. Second, the system of Dobak, III provides no means for warming the probe after it has been cooled. Many forms of cryosurgery benefit from repeated heat/thaw cycles, where the ability to heat the probe between successive cool cycles would be desirable. Further, Dobak, III proposes a probe having vacuum insulation over its exterior. The vacuum insulation, however, is shown to be on the probe only and will thus have a relatively small total volume, i.e., the volume is limited to the annular space between the coaxial members that make up the probe. Vacuum insulation in this and other systems with such low volumes suffer excessively from out-gassing (since the internal surface area from which the gas evolves is large compared to the volume available to accommodate the gas) which can lessen or destroy the insulating capability. Finally, Dobak, III proposes no means for sterilizing the probe system. The probe and compressor of Dobak, III will presumably be a closed, sealed system. The sterilization of the probe in such a system will be problematic.
For these reasons, it would be desirable to provide improved closed refrigeration systems for performing cryosurgery. In particular, it would be desirable to provide cryosurgical refrigeration systems which can be pre-cooled in a standby mode prior to use. Full probe cooling from the standby mode will preferably be achieved within several minutes or less, under thermal load. It would further be desirable to provide cryosurgical refrigeration systems which can heat the tissue-contacting probe surface between successive cooling cycles. More preferably, the ability to heat the probe will be achieved using components of a refrigeration system which are already available. Moreover, it will be desirable to provide closed cryosurgical refrigeration systems where sterilization of the probe components of such systems is facilitated, or the need to sterilize the probes is eliminated. Still further, it would be desirable to provide vacuum-insulated cryosurgical probes which are less prone to out-gassing and loss of vacuum insulation. At least some of these objectives will be met by different aspects of the present invention.
2. Description of the Background Art
U.S. Pat. No. 5,275,595 describes a cryosurgical probe which is cooled by a refrigeration system. U.S. Pat. Nos. 5,644,502 and 5,617,739 describe refrigerants and refrigeration systems of a type which could be employed as a part of the systems of the present invention. Cryosurgical treatment of the uterine endometrium and often other tissue is described in a number of patents and medical publications, including U.S. Pat. Nos. 5,647,868; 5,520,682; 5,501,681; 5,403,309; 5,400,602; 5,494,039; 5,207,674, 5,139,496; 3,924,628; and 3,889,680. Other cooled medical probes are described in U.S. Pat. Nos. 4,844,073; WO 93/06985; WO 93/08951; and WO 83/03961. Use of particulate microcrystalline material, e.g. diamond suspended in a fluorocarbon or paraffin, as a heat transfer material is described in U.S. Pat. No. 4,764,845.
The use of multi-component gas mixtures in compressor systems is described in Kleemenko (1959) Proc. 10th. Intl. Congress of Refrigeration, Copenhagen 1:34-39, Pergamon Press, London; Little (1990) Adv. Cry. Engineering 35:1305-1314 and 1325-1333.